The present invention relates to diamond-like films, articles containing diamond-like films, methods of making diamond-like films, and apparatus for depositing diamond-like films.
In recent years, materials have been developed that demonstrate a change in their optical properties on exposure to specific types of radiation. For example, some glass materials demonstrate a change in their refractive index after exposure to actinic radiation. Doping of glass fibers with germanium is one way to make them responsive to actinic radiation so that their localized refractive index can be changed.
The ability to change the optical properties of these materials, and in particular their refractive indices, has become important in numerous applications. One such application is creating gratings in optical fibers, which are regions in an optical fiber having periodic or quasi-periodic variations in refractive index. These fiber gratings can sometimes be thought of as a series of adjacent parallel planes of alternating higher and lower refractive index. Gratings have a number of important applications, including use as very narrowband retroreflectors suitable for providing feedback at a specific wavelength in fiber lasers (both in short pulse and single frequency lasers), as gain flattening devices in optical amplifiers, and as filters for multichannel wavelength-division multiplexed (WDM) communications systems.
Gratings are generally classified into two groups, long period and short period (or Bragg) gratings. Long period gratings scatter light into forward propagating cladding modes. Bragg gratings reflect light into counter propagating core (or cladding) modes.
If the spacing of the grating planes is varied across the length of the grating it is possible to produce a chirped grating, in which different wavelengths can be considered to be reflected from different points along the grating. Such gratings can be used to provide light dispersion, either to compensate for fiber dispersion in fiber links, or to manipulate optical pulses, as in a chirped pulse amplification (CPA) system.
During manufacture of optical glass fibers, the glass fibers are traditionally coated with a polymeric material to protect and maintain the intrinsic strength of the fiber during handling. The term xe2x80x9ccoatingxe2x80x9d generally refers to a material that is first applied to a solid substrate in a liquid state, then solidified by UV radiation (photopolymerizable), heat (thermoset), or by removing solvent molecules from the coating solution. In order to make a quality Bragg grating in these fibers it is usually necessary to remove the protective coating. The coating is normally removed by an acid bath. This is followed by formation of the grating and application of a new coating. This multi-step method of removing the coating, modifying the fiber, and then recoating the fiber can be time consuming, expensive and may result in a reduction in the strength of the fiber.
These steps are necessary for most applications because the gratings can not normally be formed through the coatings covering the fiber. Gratings cannot normally be formed through coatings for a number of reasons. First, the coatings often have a variable thickness, and this variable thickness can create a distorting lens that alters the path of the actinic radiation, resulting in a less precisely formed grating. Any lack of homogeneity, surface irregularities, or other optical imperfections can also degrade the quality of Bragg gratings written through such coatings. Second, although some coatings are highly transparent, they still often partially absorb the actinic radiation and overheat or are degraded by the high doses of radiation energy typically needed to form Bragg gratings in photosensitive glasses. In some circumstances, irradiation can actually result in the coating being degraded (such as by being charred) or ablated from the fiber.
A need exists for an improved protective layer for application to substrates, including substrates that may be altered by actinic radiation. The layer of material should preferably protect and retain the initial strength of the substrate, particularly when the substrate is an optical glass fiber. The protective layer should also allow passage of actinic radiation into the substrate, such as actinic radiation into optical glass fiber. In addition, it is desirable that the protective layer be such that it can be applied in a substantially uniform layer in order to control the distortion and refraction of the actinic radiation as it is directed into the substrate.
The present invention is directed to articles having a diamond-like film, methods of making the articles, and apparatus for making the articles. In specific implementations, the articles include a glass substrate with a layer of a diamond-like film. The glass substrate is optionally capable of demonstrating a change in physical properties such as refractive index upon exposure to actinic radiation, and the diamond-like film is a substantially amorphous film that allows passage of actinic radiation into the glass substrate. In this manner, the diamond-like film is xe2x80x9cwrite-throughxe2x80x9d because it allows passage of the radiation into the substrate in order to generate changes in the substrate properties (such as the refractive index).
The diamond-like film is suitable, for example, for deposit on glass fibers, including optical glass fibers used to transmit data. The film provides protection for the glass fiber substrates and avoids lowering of the fibers"" strength below acceptable levels during handling in the writing process. In most applications, the film enhances the strength of the fibers relative to uncoated fibers subjected to the same writing and handling conditions. In addition, the film can be formed on the substrate in a highly uniform manner that provides improved optical and physical properties for the finished article.
The articles made in accordance with the present invention include articles suitable for use in making Bragg gratings. The write-through characteristics of the film, along with its generally uniform thickness, allow for the formation of high quality Bragg gratings that can be formed quickly and with great precision. In addition, the methods of making Bragg gratings in accordance with the invention permit the strength of the fibers to be substantially preserved, and even enhanced, compared to fibers that have not been modified in accordance with the invention.
In order to provide an adequate film for write through applications the diamond-like film preferably allows transmission of radiation without degradation of the film. If any degradation does occur in the film, it is preferred that the degradation is insufficient to cause diminished strength properties of the substrate. The film preferably remains deposited on the substrate and is still able to be written through after exposure to light from a frequency doubled Argon laser operating at writing beam power densities of 4000 W/cm2 at a wavelength of 244 nm for one hour. A frequency doubled Argon laser is a continuous laser based on a laser cavity containing ionized argon gas and a crystal that doubles the frequency and changes the wavelength of output radiation for example from 488 nm to 244 nm.
Various diamond-like films are suitable for the present invention, including diamond-like films selected from the group including diamond-like carbon, diamond-like glass, diamond-like networks, and interpenetrating diamond-like nanocomposites. In specific implementgations of the invention the diamond-like film has on a hydrogen-free basis at least 30 atomic percent carbon, from 0 to 50 atomic percent silicon, and from 0 to 50 atomic percent oxygen. The diamond-like film typically includes on a hydrogen-free basis at least 25 atomic percent carbon, from 0 to 50 atomic percent silicon, and from 0 to 50 atomic percent oxygen; and in certain implementations the film include from about 30 to about 100 atomic percent carbon, from about 20 to about 40 atomic percent silicon, and from about about 30 to about 40 atomic percent oxygen. In specific advantageous implementations the diamond-like film includes from 30 to about 36 atomic percent carbon, from 26 to 32 atomic percent silicon, and from 35 to 41 atomic percent oxygen on a hydrogen free basis. xe2x80x9cHydrogen-free basisxe2x80x9d refers to the atomic composition of a material as established by a method such as Electron Spectroscopy for Chemical Analysis (ESCA), which does not detect hydrogen even if large amounts are present in the thin films.
Thin films made in accordance with the invention may have a variety of light transmissive properties. Thus, depending upon the application, the thin films may have increased transmissive properties at various frequencies. However, in specific implementations the thin film is at least 50 percent transmissive to radiation at one or more wavelength from about 180 to about 800 nanometers. In particular, the thin film should be transmissive to light at the wavelength of the actinic radiation used to alter the refractive index or other properties of the substrate. In other advantageous implementations the film is transmissive to greater than 70 percent (and more advantageously greater than 90 percent) of radiation at one or more wavelengths from about 180 to about 800 nanometers. High transmissivity is typically preferred because it allows thicker films to be produced without significant reduction in radiation intensity passing through the film. Thus, although the present invention is suitable for lower transmissivity, particularly desirable levels are greater than 50 percent.
The invention is further directed to a method of depositing a diamond-like film onto a substrate, including the deposit of a write-through diamond-like film onto a glass substrate. The method includes providing a capacitively coupled reactor system having two electrodes in an evacuable reaction chamber. The chamber is partially evacuated, and radio frequency power is applied to one of the electrodes. A carbon-containing source is introduced between the electrodes to form a plasma including reactive species in proximity to the electrodes, and to also form an ion sheath proximate at least one electrode. The glass substrate is placed within the ion sheath and exposed to the reactive species to form a diamond-like thin film on the substrate. The conditions can result in a thin film that includes, for example, a diamond-like film including on a hydrogen-free basis at least 25 atomic percent carbon, from 0 to 50 atomic percent silicon, and from 0 to 50 atomic percent oxygen. The thin film can be made to a specific thickness, typically from 1 to 10 microns.
As used herein, the term xe2x80x9cdiamond-like filmxe2x80x9d refers to substantially or completely amorphous films included of carbon, and optionally including one or more additional components selected from the group including hydrogen, nitrogen, oxygen, fluorine, silicon, sulfur, titanium, and copper. Other elements may be present in certain embodiments. The films may be covalently bonded in a random system or in an interpenetrating system, such as in an interpenetrating diamond-like nanocomposite (called DYLYN), as described, e.g., U.S. Pat. No. 5,466,431. The amorphous diamond-like films of this invention may contain clustering of atoms that give it a short-range order but are essentially void of medium and long range ordering that lead to micro or macro crystallinity which can adversely scatter actinic radiation having wavelengths of from 180 nm to 800 nm.
As used herein, the term xe2x80x9cparallel plate reactorxe2x80x9d means a reactor containing at least two electrodes, wherein the primary mechanism for current flow between the electrodes is capacitive coupling. The electrodes may be asymmetric, meaning that they may be of different size, shape, surface area, etc. and need not necessarily be parallel to each other. One electrode may be grounded, and one electrode may be the reaction chamber itself.
As used herein, the term xe2x80x9camorphousxe2x80x9d means a substantially randomly-ordered non-crystalline material having no x-ray diffraction peaks or modest x-ray diffraction peaks. When atomic clustering is present, it typically occurs over dimensions that are small compared to the wavelength of the actinic radiation.
As used herein, the term xe2x80x9cplasmaxe2x80x9d means a partially ionized gaseous or fluid state of matter containing reactive species which include electrons, ions, neutral molecules, free radicals, and other excited state atoms and molecules. Visible light and other radiation are typically emitted from the plasma as the species including the plasma relax from various excited states to lower or ground states. The plasma usually appears as a colored cloud in the reaction chamber.
As used herein, the term xe2x80x9cnegative biasxe2x80x9d means that an object (e.g., an electrode) has a negative electrical potential with respect to some other matter (e.g., a plasma) in its vicinity.
As used herein, the term xe2x80x9cnegative self biasxe2x80x9d, with respect to an electrode and a plasma, means a negative bias developed by application of power (e.g., radio frequency) to an electrode that creates a plasma.
Advantages of the invention will be apparent from the following description, figures, examples, and the appended claims.